Wavelength division multiplexing (WDM) systems typically comprise multiple separately modulated laser systems at the transmitter. These laser systems are designed or actively tuned to operate at different wavelengths. When their emissions are combined in an optical fiber, the resulting WDM optical signal has a corresponding number of spectrally separated channels. Along the transmission link, the channels are typically collectively amplified in semiconductor amplifier systems or gain fiber, such as erbium-doped fiber and/or regular fiber, in a Raman amplification scheme. At the receiving end, the channels are usually separated from each other using, for example, thin film filter systems to thereby enable detection by separate detectors, such as photodiodes.
The advantage of WDM systems is that the transmission capacity of a single fiber can be increased. Historically, only a single data channel was transmitted in each optical fiber. In contrast, modem WDM systems contemplate hundreds of spectrally separated channels per fiber. This yields concomitant increases in the data rate capabilities of each fiber. Moreover, the cost per bit of data in WDM systems is typically less than comparative non-multiplexed systems. This is because optical amplification systems required along the link are amortized across all of the separate wavelength channels transmitted in the fiber. With non-multiplexed systems, each channel/fiber would require its own amplification system. As an additional functionality, different wavelengths in WDM optical networks can also be used for wavelength routing of optical signals. This is sometimes referred to as metro WDM.
Nonetheless, there are challenges associated with implementing WDM systems. First, the transmitters and receivers are substantially more complex since, in addition to the laser diodes and receivers, optical components are required to combine or multiplex, the channels into, and separate, or demultiplex, the channels from, the WDM optical signal. Moreover, there is the danger of channel drift, where the channels lose their spectral separation and thus overlap each other. This interferes with channel demodulation at the receiving end.
Minimally, the optical signal generators, e.g. the semiconductor laser systems that generate each of the optical signals corresponding to the optical channels for a fiber link, must have some provision for wavelength or frequency control. Generated and transmitted optical channel wavelength should be controlled to within a fraction of the channel wavelength spacing. Especially in systems with center-to-center channel wavelength spacings of less than 1 nanometer (nm), the optical signal generator must have a precisely controlled carrier wavelength. Any transmitted wavelength wander impairs the demodulation of the wandering signal at the far end receiver, since the wavelength is now different from that expected by the corresponding optical demultiplexer and signal detector. The wandering wavelength signal can have decreased power in its intended receiver; it can also interfere with and impair the demodulation of spectrally adjacent channels when their spectrums overlap each other.
Frequency/wavelength lockers are typically used in semiconductor based laser systems to detect the wavelength of operation of the laser relative to the intended ITU grid channel center, for example, and then generate a feedback signal that can be used as a tuning signal to control the laser""s wavelength of operation. Typically, lockers are used to stabilize the operating wavelength of the semiconductor laser either to a single channel wavelength, or they can allow the laser to hop between operational channel wavelengths.
Lockers have been proposed that are based on etalons. Typically, the thickness of the etalons must be controlled to define the locker""s frequency locking range and the spacing of the frequency locked channels, which is given by the etalon free spectral range (FSR). Further, the etalons reflectivities must be controlled in order to achieve the desired amount of wavelength discrimination.
One of the problems associated with etalon-based lockers, however, surrounds manufacturability. The thickness of the etalon must be controlled to absolute tolerances to control FSR and to align etalon resonances precisely with the standard channel frequencies, with some adjustment allowed by angle tuning. Moreover, the reflectivities must be selected and controlled to achieve the desired level of discrimination. Further, it is not uncommon in these locker systems to use multiple etalons. The thicknesses of these etalons and their reflectivities must also be controlled, typically, on a relative basis.
A further problem arises with the basic filtering characteristic of etalons. Generally, an etalon""s frequency response is defined according to the function (1+sin2x)xe2x88x921. Within a period of operation of the etalon, the filtering characteristic appears as a bandpass filter. Such devices provide good discrimination on either side of the center bandpass frequency where large changes in transmission result from changes in input optical frequency. However, these devices perform relatively poorly when transmission slope with frequency is low, which is the case near the peaks and broad valleys of the filter transmission function. This can become a problem when wavelength control is required over a relatively large band, as required in some tunable laser systems, such as those used in wavelength division multiplexing systems.
Some have proposed etalon-based systems that have wider bandwidths of operation. This can be achieved by one of two approaches. First, multiple etalons can be used. The problem here, however, is that these multiple etalons must be manufactured with close tolerances relative to each other. Alternatively, low-finesse etalons can be used, thereby providing transmission changes over a wider frequency range. The problem arises, however, that contrast is poor in such low finesse devices. Another disadvantage and strong limitation of etalon lockers is that while they lock the laser frequency near one of the periodic etalon resonances corresponding to the frequency grid channels, they provide no information whatsoever as to which particular channel on the grid the laser is locked to.
These problems with conventional etalon lockers create problems with newer dense WDM (DWDM) systems. The channel frequency spacing can be tight, 100 GigaHertz (GHz) to as tight as 50 GHz and 25 GHz in some currently proposed systems. Further, the number of potential channels on a link can be large. Observation of the ITU frequency grid suggests 100""s of channels on a link in the Lxcex1, Cxcex1, and Sxcex1 bands, even if the 50 GHz offset of the Lxcex2, Cxcex2, and Sxcex2 band is ignored.
The present invention is directed to wavelength measurement and control system that is based upon a Mach-Zehnder type, or interferometric, filter made using birefringent material, in the preferred implementation. The advantage of this birefringent material filter is that it has a substantially sinusoidal spectral response. As a result, the responses of multiple birefringent filters can be combined to yield a filter system that has an additive wavelength resolution that is uniform and spectrally stable for all wavelengths. That is, the wavelength measurement and control system does not have regions where wavelength resolution is degraded.
In general, according to one aspect, the invention features a wavelength measurement system. This system comprises a birefringent, waveplate, filter system that applies multiple spectral filtering characteristics to an input beam. Multiple detectors are used to detect the beam after being filtered by the multiple spectral filtering characteristics. Finally, a controller determines the wavelength of the input beam in response to the relative responses of the multiple detectors.
Preferably, the birefringent filter system comprises several blocks of birefringent material with different functions. In one implementation, it comprises birefringent waveplates placed between two blocks of birefringent polarizing beam splitters. Specifically, two waveplates are used in parallel to each other between the birefringent displacer/polarizer material blocks.
In general, according to another aspect, the invention also features a semiconductor laser system with wavelength control. This system comprises a semiconductor laser system that generates a polarized output beam. A wavelength detection system, on the output of the laser, comprises a birefringent filter system and multiple detectors for detecting the beam after being filtered by multiple spectral filtering characteristics of the filter system. A controller adjusts the wavelength of operation of the semiconductor laser system in response to the responses from the multiple detectors.
A feature of some embodiments of this invention concerns the fact that, generally, the birefringent filter systems, while being simple, are typically sensitive to the polarization of the input light. Semiconductor lasers typically produce polarized light as an artifact of their construction; also, polarized light from such lasers is typically required for polarization sensitive external modulators, such as lithium niobate modulators. Depending on the selected tensile or compressive strain characteristics of the semiconductor epitaxial material, the polarization of the light from the laser system is specified. Thus, the precondition of known polarization is provided as a consequence of using the semiconductor laser system.
In general, according to still another aspect, the invention features a wavelength detection system. This wavelength detection system comprises a filter system that provides multiple spectral filtering characteristics to an input beam. These multiple filtering spectral characteristics preferably have a spectrally stable additive wavelength resolution. The advantage of such a stable additive wavelength resolution is the fact that the system can resolve wavelengths across some spectral band with consistent accuracy. Multiple detectors are then used to detect the beam after being filtered by the multiple spectral filtering characteristics.
An important advantage of the present invention is that optical frequency measurement is periodic with the input optical frequency; the measurement period being given by the constituent filter free spectral range. This frequency measurement range can be chosen to be the full optical band of interest, such as the C band, or only a fraction thereof. For a particular frequency meter in accordance with the present invention, frequency measurement has a constant, or near constant, frequency resolution over the full free spectral range; in addition, center frequency of this measurement range can be chosen arbitrarily. Therefore, the same device, perhaps with adjusted software calibration, can be used in the C, L, S or other optical communications wavelength band. Also, for a continuously tuned laser source, such a frequency meter can track the source optical frequency over an arbitrarily wide frequency range through the multiple periods of the meter response. Importantly, frequency meter and controller in accordance with the present invention can measure and lock a laser to an arbitrary frequency over the device measurement range. Unlike other etalon based wavelength lockers, our device is not limited to a predetermined fixed grid of equally-spaced frequencies.
The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention.